1. Field of the Invention
The present invention relates to a camshaft for controlling valves in internal combustion engines. The camshaft includes a shaft and at least one cam slid onto and connected to the shaft. The cam has a recess for receiving the shaft. The recess has at least one radially inwardly extending projection which engages a corresponding groove in the shaft.
The present invention also relates to a method of manufacturing the above-described camshaft.
2. Description of the Prior Art
Camshafts are known in which the cams and the shafts are manufactured separately and are then joined to form a structural unit. In a known construction of this type, for example, from German Offenlegungsschrift No. 28 38 995 or German Offenlegungsschrift No. 33 21 846, the shaft has a plurality of axially extending grooves and the bore of the cam has a corresponding radial projection which engages this groove. The bore of the cam surrounds the shaft with play, however, the above-mentioned projection has radially such a size that the cams and the shafts can be joined with frictional engagement and remain in engagement. After the cams and the shafts have been joined, the gap existing as a result of the above-mentioned play is filled from the outside with hard solder. The number of grooves provided circumferentially on the shaft is provided as required by the phase relation of the cams for the respective use of the camshaft. This known construction is not advantageous because a complicated operation is required for manufacturing the grooves on the shaft in the correct positions. Thus, the grooves must be manufactured very exactly and they must also be exactly positioned with respect to their angular positions.
From German Offenlegungsschrift No. 32 27 693 it is also known to slide a finished cam onto a finished hollow shaft in the correct position and to widen the hollow shaft from the inside in order to obtain a fixed connection between the cams and the shafts. As a result, a press fit between the hollow shaft and the individual cams is obtained. For increasing the press fit, the hollow shaft is filled with plastics material and the hollow shaft becomes a load-bearing composite structural component by pressing in the plastics material. The pressure required for widening the hollow shaft is generated by pressing in the plastics material. This method is very complicated. In addition, the tensions acting on the cams are extremely great due to the necessary press fit because the cams are exclusively held on the shaft through frictional engagement. Accordingly, it is not possible to use cams which are manufactured by sintering because sintered cams cannot absorb the occurring tensions over time without suffering damage. Forged or rolled cams may be capable of absorbing the occurring tensions without reducing the service life thereof. However, the manufacture of the cams becomes complicated when the cams are manufactured by forging or rolling.
The camshafts manufactured in accordance with German Offenlegungsschrift No. 25 46 802 also includes a hollow shaft. Portions of the hollow shaft are widened by inserting a rubber rod whose diameter corresponds to the inner diameter of the hollow shaft and which is pressed together from the opposite side of the hollow shaft. For the above-mentioned reasons, this type of camshaft has not been found useful.
Instead of widening portions of the hollow shaft from the inside, it is also known from German Offenlegungsschrift No. 33 23 640 to provide radial projections on the bore surfaces of the cams and to select the diameter of the hollow shaft in such a way that the cams can be slid onto the shaft and positioned on the shaft without requiring significant force. Subsequently, the shaft is expanded over its entire axial length by means of a mandrel which is pulled through the hollow shaft. For reasons of stability, the hollow shafts used in this case have a substantial wall thickness, so that the widening of the shaft, whether carried out over portions thereof or over the entire length thereof, requires substantial deforming forces which also influence the initial geometry of the hollow shaft. This is a disadvantage because it is the present tendency that sintered cams are used which are extremely accurate to size and do not receive further surface finishing. However, this requires that the hollow shaft has geometrically accurate dimensions.
German Offenlegungsschrift No. 31 28 522 describes a camshaft in which the cam is tangentially adjustable relative to the shaft. Also in this case, the cam is a separately manufactured component and is fastened on the shaft by means of a sleeve with a press fit. The sleeve has a cylindrical bore which corresponds to the diameter of the shaft. The outer surface of the sleeve is truncated-cone shaped which corresponds to a similar conical bore in the corresponding cam, wherein a compressed oil connection is provided in each cam in the conical bore in the region of its greatest diameter. Another compressed oil connection is provided in the shaft next to the respective cam, wherein this compressed oil connection leads to the inner bore of the sleeve through a compressed oil duct. As a result of the feature, no axial forces, i.e., forces acting in the direction of the shaft, occur when the connection between the shaft and the sleeve with cam is separated by exerting pressure on the compressed oil connection. As a result, after the cam including sleeve have been separated, the cam can be rotated, i.e., can be adjusted in tangential direction. This can be done without any additional undesired adjustments which would later have to be reversed. Thus, it becomes unnecessary to once again slide the cam onto the sleeve.
Finally, it should be noted that it is known from German Offenlegungsschrift No. 21 59 264 to connect a flange with a hub by means of chip-removing pressing the flange onto the hub by mutually sliding the flange provided with a toothing onto a support shoulder of the hub. Compared to the flange, the hub has a short axial length, with only one flange being arranged on the hub.
A camshaft is a structural machine element of a special type because it is subjected during operation to very extremely dynamic stresses. Accordingly, in all known camshafts, a material-locking and/or positively locking connection between the shafts and cams is present. If the cams are to be securely fixed on the shaft exclusively by means of a press fit, the press-fit connection must be very strong which means that extremely high static loads act on the cam onto which additional dynamic loads are superimposed during operation. Cams which are manufactured by sintering are unable to absorb without damage loads of this type over a longer period of time. This is true at least for sintered cams as they are manufactured in accordance with the present state of the manufacturing art and the materials for sintering purposes available today.
It is, therefore, the object of the present invention to provide a camshaft in which sintered cams are used, while the necessary radial pressure between cam and tube is only of such a magnitude that loads occur which can be absorbed by the sintered cam over a long period of time. Still, the support between the shaft and the cams is to be reliable and secure.